1. Field of the Invention
The present invention relates to a pixel structure of an organic electroluminescent display panel and a method of making the same, and more particularly, to a pixel structure having a plurality of luminescent devices connected in series in sub-pixel regions and a method of making the same.
2. Description of the Prior Art
Organic electroluminescent displays, such as organic light emitting diode (OLED) displays, have advantages of small size, high resolution, high contrast ratio, low power consumption, and active luminescence, which put the organic electroluminescent displays in position to surpass liquid crystal displays as the next generation flat panel display technology.
Please refer to FIG. 1 and FIG. 2. FIG. 1 is a schematic diagram illustrating a structure of a single luminescent device of a conventional organic electroluminescent display panel, and FIG. 2 is a schematic diagram illustrating a driving circuit structure of a pixel structure of a conventional organic electroluminescent active-matrix display panel. As shown in FIG. 1, a luminescent device of the conventional electroluminescent display panel includes a substrate 10 and an anode 12, a hole injection layer 14, a hole transport layer 16, an organic luminescent layer 18, an electron transport layer 20 and a cathode 22, which are disposed on the substrate 10 in that order. The stacked structure described above is a commonly-used structure in the formation of luminescent devices, and luminescent devices utilizing the above-mentioned stacked structure are generally called “normal-type” luminescent devices. Fabrication of the normal-type luminescent device is a mature technology, and has advantages of high yield and high reliability.
Early pixel structures employing the normal-type luminescent device have the following shortcomings when operating with a-Si TFTs produced in an amorphous Si process. As shown in FIG. 2, a driving circuit of the pixel structure employing the normal-type luminescent device includes two thin film transistors T1, T2 and a capacitor C. The thin film transistors T1, T2 can be NMOS thin film transistors. The gate electrode of the thin film transistor T1 is connected to a scan line, and a source electrode and a drain electrode are respectively connected to a data line and a gate electrode of the thin film transistor T2. A source electrode of the thin film transistor T2 is connected to a voltage source Vdd, and a drain electrode of the thin film transistor T2 is connected to an anode 12 of the luminescent device. As shown in FIG. 2, if the NMOS produced by the normal amorphous Si process is used, and the luminescent device is located between the thin film transistor T2 and the voltage source Vss, the disposition affects the threshold voltage of the thin film transistor T2, due to increased device voltage during operation of the luminescent device, so that the current of the thin film transistor T2 becomes unstable. Therefore, in the thin film transistor process of the driving circuit, a PMOS produced by a low temperature poly-silicon (LTPS) process is used to lower the effect of the increased voltage across the luminescent device. However, the LTPS process is more complicated, and panel uniformity suffers. Development of a large substrate is not mature.
Additionally, the single-pixel structure of the conventional electroluminescent display panel only has a single luminescent device, so the driving current of the luminescent device is larger. The large current not only affects the driving stability of the thin film transistor T2, especially for the amorphous thin film transistor with low electron mobility, but also increases power consumption and generates higher thermal energy, which affects the lifetime of the luminescent device.